"81" series are color corrective filters that 'warm' bluish tones. They are avaible in several strengths (A, B, C & EF) with 'A' being the mildest warming effect.
•Available in all popular sizes.
Reviewed by 1 customers
It works just fine. No added distortion. Effective. It's a bit thicker than some of Hoya's other filters, but with a DX/1.6x camera, you won't notice. I can stack 3 of them w/o vignetting on an 18mm lens. Not a problem.